1. Field of the Invention
The present invention relates to the preparation of direct methanol fuel cells. More specifically, the present invention relates to methods for conditioning the catalysts used in the membrane electrode assembly of direct methanol fuel cells to reduce the amount of surface oxides, and thus improve the electrooxidative activity of the membrane electrode assembly.
2. Description of Related Art
Production of sufficient electrical power to meet the needs of a growing population and economy is a constant challenge. In view of limitations on traditional electric power production, there is increased interest in alternative means of producing electricity.
One technology that has evoked increasing excitement in the area of alternate energy sources in recent years is the fuel cell. Fuel cells are devices that generate electricity directly from chemical energy. Fuel cells are structurally similar to some batteries, having an anode, a cathode, and an electrolyte. Unlike batteries, however, fuel cells are supplied with a continuous stream of fuel and oxidant. The fuel is supplied to the anode, and the oxidant is supplied to the cathode. The fuel and oxidant are electrochemically combined, thus releasing electrical energy, which is available for use.
Fuel cell electrodes often comprise a porous electrically conductive substrate on which an electrocatalyst is deposited. In many newer fuel cells, the electrolyte is often a solid polymer to which the electrodes are attached, thus forming a membrane electrode assembly. The electrolyte used may be a solid polymer electrolyte, also referred to as an ion exchange membrane, disposed between the two electrode layers. Flow field plates for directing the reactants across a surface of each electrode may also be included in the membrane electrode assembly.
Many types of electrocatalysts may be used on the electrodes of the fuel cell, including metal blacks, metal alloy blacks, or supported metal catalysts. Electrocatalysts such as these are generally attached to the electrode as a layer applied to either an electrode substrate or to the membrane electrolyte itself. The electrocatalyst may be applied by mixing fine electrocatalyst particles with a liquid, thus forming an ink, which is then applied to the substrate. This ink preferably wets the substrate surface, but does not penetrate too deeply, so as to keep as much catalyst as possible at the interface between the electrolyte and the electrode.
Proper application of electrocatalyst renders it accessible to reactants, electrically connected to current collectors associated with the fuel cell, and ionically connected to the electrolyte. In operation, electrons and protons are generated at the electrocatalyst of the anode. From here, the electrons are channeled through the current collectors to an external circuit, thus producing a useful electric current. The protons, meanwhile, are conducted through the electrolyte to the cathode of the fuel cell.
The mechanism of energy production seen in fuel cells sets them apart from other energy production technologies in that it provides a very efficient, clean, and quiet source of energy. Specifically, since fuel cells effectively convert chemical energy to electricity, without the intermediate steps of conversion to heat and subsequent conversion to mechanical energy common to most energy production methods, efficiency is increased. This is due to the fact that conversion of heat to mechanical energy is associated with limited efficiency. Further, since no combustion takes place in the energy conversion process in a fuel cell, the chemical products of the fuel cell can be more accurately predicted and carefully chosen. Indeed, in many fuel cell designs, the main product of the reaction is selected to be water vapor.
Electrochemical fuel cell performance may be judged by the voltage output from the cell for a given current density. Higher cell performance is correlated with a higher voltage output for a given current density or higher current density for a given voltage output. Substantial improvement in the performance of a fuel cell may be obtained by improving the utilization of the electrocatalyst. By doing so, the same amount of electrocatalyst may cause a much higher rate of chemical conversion, thus improving the efficiency of the fuel cell.
Several substantial barriers stand in the way of full-scale implementation of fuel cells as the power supply for homes, automobiles, and businesses. First, fuel cells are currently expensive when compared with traditional energy sources. Furthermore, there is no ready infrastructure for supplying fuel to fuel cell devices. Additionally, many engineering and safety difficulties must still be fully resolved before regulators will permit fuel cells to be used to power automobiles and other vehicles.
Some of the engineering and safety issues are faced in connection with the use of hydrogen as a fuel for the devices. Others are faced during the construction/preparation of the fuel cells themselves. Hydrogen is difficult to store, especially in vehicles. As a result, efforts have progressed to develop fuel cells capable of operating on alternative fuels which either may be reformed to provide hydrogen, or which may be used directly. Additionally, the use of hydrogen in fuel cell fabrication endangers workers and production facilities. It is thus desirable to provide production methods which do not require the use of hydrogen gas, thus sparing added expense and rendering the production process much more safe.
Direct methanol fuel cells (“DMFCs”) are fuel cells that operate by directly electrochemically oxidizing methanol at an anode electrocatalyst. This anode reaction produces carbon dioxide, protons, and electrons. This type of fuel cell has begun to gain popularity since it does not require the use of gaseous hydrogen as a fuel. In the reaction, the electrons are channeled from the anode, where they are produced, through a circuit external to the fuel cell, to the cathode electrocatalyst. At the cathode, electrons recombine with protons and oxygen to form water. As noted above, often in such fuel cells, the electrolyte is a polymer electrolyte membrane. These membranes allow larger convenience in fuel cell design and enable operation with distilled water as the only liquid in the cell, other than the fuel itself.
Direct methanol fuel cells are an improvement over the current art in that they are capable of using methanol as a fuel instead of gaseous hydrogen. Further, the methanol may be used directly without first being processed in a reformer to generate the needed hydrogen. This eliminates the added weight and expense that a reformer adds to a design.
In addition, as noted above, hydrogen may be required in the manufacturing of fuel cells, including direct methanol fuel cells. One example of this is the use of hydrogen in conditioning the electrocatalysts of a direct methanol fuel cell, especially at the anode. This conditioning step is included to facilitate the reduction of any surface oxides found on the electrode. It has been discovered that fuel cell efficiency is increased when the platinum/ruthenium (PtRu) anode catalyst is conditioned to remove surface oxides as much as possible. Specifically, x-ray photoelectron spectroscopy (XPS) demonstrated that an increased metallic content of the Pt/Ru catalyst aids in methanol electrooxidation activity. Wieckowski et al., J. New Mat. Electrochem. Systems, 3:275-284 (2000).
A current laboratory method of conditioning the membrane electrode assemblies (or “MEAs”) used in direct methanol fuel cells involves flowing hydrogen gas over the anode side of the MEA at elevated cell temperatures (such as 80° C.). During this process, the cell voltage is held at 0.6 V until the current reaches a steady state. Oxides at the anode surface are reduced by the hydrogen gas, thus rendering a more active electrocatalyst.
As briefly noted above, however, hydrogen gas is hazardous. Its use requires precautions that may be cost prohibitive, while still remaining a potential danger to employees and a liability to manufacturers. As a result, it would be an improvement in the art to provide alternative conditioning procedures, which improve the electrocatalytic function of the membrane electrode assembly of a direct methanol fuel cell without dependency on gaseous hydrogen.
Accordingly, a need exists for methods of conditioning the PtRu anode of direct methanol fuel cells, which do not use hydrogen gas, but which effectively and efficiently reduce surface oxides found at the Pt/Ru anode, thus increasing the activity and/or efficiency of the direct methanol fuel cell. In accordance with the present invention, n addition to the beneficial effects of effective reduction of the DMFC anode catalyst as conditioning at the beginning of cell operation, in-situ reduction of the anode catalyst surface can also be beneficial as DMFC conditioning step following long-term DMFC cell operation. Long term DMFC performance decay can be caused by a higher state of surface oxidation of the PtRu anode catalyst, gradually developing during cell operation as the anode experiences higher potentials. Brief application of effective anode surface reduction in-situ conditioning will enable cell performance recovery.